3D Seismic Interpretation of Stratigraphic Horizons and Structures in Time Domain Jura Ost
Nagra (2024): 3D Seismic Interpretation of Stratigraphic Horizons and Structures in Time Domain Jura Ost . Nagra Arbeitsbericht NAB 23-17.
A 3D seismic evaluation was carried out for the Jura Ost (JO), Nördlich Lägern (NL) and Zürich Nordost (ZNO) siting regions in the course of the repository site selection process. The aim is to identify and map the most undisturbed area for each of these three siting regions. It is important to highlight that this report focuses on seismic-scale observations. The development of geological concepts and the abstraction to subseismic scale interpretation is part of a multi-scale and multi-disciplinary integration and is captured in NTB 24-17 (Nagra 2024d).
While 3D seismic data greatly improve the imaging of the subsurface, the challenge of separating seismic reflections associated with geological features and seismic artefacts represents the key pitfall in the interpretation results, especially for onshore data with complex and heterogeneous shallow geology. Interpreting 3D seismic data is therefore a complex process that involves multiple steps. These can include some looping or repetition of the workflow steps, and require a deep understanding of geophysics, geology and seismic data processing.
In addition, for the purpose of site selection, emphasis was placed on the comparability of the seismic interpretation results. Despite being located in the Molasse Basin and being ~20 km apart, the Jura Ost, Nördlich Lägern and Zürich Nordost siting regions are demonstrably characterised by a different subsurface architecture. Therefore, to ensure a comparable and balanced evaluation, not only was seismic data reprocessing designed such that all the processing input parameters and workflows were consistent across the three seismic surveys, but also an interpretation approach with rigorous criteria was established and implemented in the interpretation of the seismic surveys, covering the three siting regions. Consequently, importance was given to the methodology section, which is repeated in the three seismic interpretation reports for the respective siting regions (NAB 23-17, NAB 23-18 and NAB 23-19), where the applied workflow is illustrated based on site-specific examples.
The resulting horizon - fault framework in JO siting region includes the interpretation of 7 key horizon markers:
(i) 8 Oxf 700 – Intra Wildegg
(ii) 8 Oxf 975 – Top Dogger
(iii) 9 Aal 300 – Top Opalinus
(iv) 9 Sin 200 – Near Top Klettgau
(v) 227 Car 200 – Near Top Bänkerjoch
(vi) 237 Lad 200 – Intra Bänkerjoch
(vii) 9 Per 100 – Near Base Mesozoic Unconformity
and 3D fault interpretation in the form of fault sticks / planes.
The seven horizons were selected to represent key surfaces for the definition of the host rock (Opalinus Clay) as well as the upper and lower confining geological units, while ensuring interpretability (i.e. imaging continuity) to reduce the picking uncertainty across the structurally complex zones.
Confidence in the interpretation results was established by an in-depth analysis of the uncertainties associated with the interpretation of seismic data. Uncertainties associated with data quality were mitigated by the use and comparison of all the seismic processing vintages, while the impact of human bias was assessed by superimposing all the available interpretations from different interpreters. Different seismic processing datasets interpreted by different interpreters resulted in a technically robust, consistent and comparable image of the three investigated siting regions.
The evaluation performed shows that the Jura Ost siting region can be subdivided into four main structural domains: (i) Northern Fold-and-Thrust Zone (NFTZ, i.e. Mandach Thrust), (ii) Southern Deformation Zone (SDZ, including structures such as the Brugg Structure Zone, Gallenkirch Structure Zone, Villnachern Fault), (iii) the Effingen Fault Zone (EFZ) and (iv) the Central Panel (CP, which includes the Siggenthal Anticline). Seismically undisturbed areas can be recognised across the Central Panel and the northern sector of the Southern Deformation Zone. If only faults affecting the lower Mesozoic succession (i.e. faults tipping up below the 174.7 Toa 100 – Top Lias) are considered, a large undisturbed area with an extent of 31 km2 can be delimited.
A divergence in the fault distribution and density is observed when comparing seismic-scale faults mapped in 2023 (this report) with previously proposed structural maps (Nagra 2022, NAB 18‑34). Such a divergence is directly connected to the distinction between structural lineaments proposed for the ASR mapping, where “structural” seismic discontinuities (deformation zones) are undifferentiated between faults and folds, and when considering type C faults (attribute response only) from the 2018 interpretation (Nagra 2019). The comparison considering only type A faults (fault interpreted both on amplitude sections and attribute slices: 2018 and 2022 interpretation vintage) results instead in a consistent image with the proposed interpretation of the fault distribution in the JO siting region, suggesting that the observed difference in the seismic fault pattern is controlled by attribute-derived lineaments and their geological interpretation.